When Harry and Hagrid are escaping from Privet Drive, the pursuing Death Eaters determine that he is the real Harry Potter, out of the six sets of protector and Potter there. Having discovered this, Lord Voldemort gathers his Death Eaters and attacks Harry personally. Harry's wand destroys the wand Voldemort is using (which he had taken from Lucius Malfoy), so Voldemort demands a fresh wand from Selwyn. Before he can use it, though, Harry is through the protective spells surrounding Ted Tonks' house.

Hermione, in disguise as Mafalda Hopkirk in the Ministry, catches sight of the Locket Horcrux that Dolores Umbridge is wearing, and comments that it is lovely. Umbridge says that the S is for Selwyn, and she wears it because she is related to the Selwyn family, one of the last remaining bastions of pureblood wizardry.

When Xenophilius Lovegood, in hopes of winning his daughter's freedom, summons the Death Eaters, telling them that Harry is in his house, Selwyn is one of the two Death Eaters who respond to the summons. He seems annoyed at the summons, saying that this is the third time Xeno has called, and also because he had apparently tried to blow them up as they arrived. While he accuses Xeno of making things up again, he is also repeatedly jinxing him. Hermione engineers their escape by disapparating the Trio in midair as they fall through the hole she makes in the ceiling, allowing the Death Eaters to see her and Harry so that they know Xeno isn't lying this time, but hiding Ron as he is supposed to be ill with Spattergroit at The Burrow.

Selwyn is a Death Eater and has a connection with Voldemort because of this. Dolores Umbridge claims a family connection, but there is no real evidence of this; it is, in fact, suggested that Umbridge is trying to manufacture evidence of a familial connection that does not exist.